(1) Field of the Invention
The present invention relates to a device for damping a rotor, and also to an associated rotor and aircraft.
The invention relates in particular to rotorcraft rotors comprising a hub that is driven in rotation about an axis of rotation by an outlet shaft from a main gearbox, together with a plurality of blades that are fastened to the hub by means of appropriate hinges. Such rotors are said to be “articulated rotors”.
(2) Description of Related Art
The oscillations of each blade about its lead-lag axis can become coupled in unstable manner with the movements or the elastic deformation modes of the airframe, with the lead-lag oscillations of a blade being the result of the natural response and of the forced response of said blade. In particular, the oscillations of a helicopter standing on the ground on its landing gear while its rotor is turning can give rise to a phenomenon known as “ground resonance”.
During lead-lag excitation of the blades of a rotor, the blades move away from their equilibrium positions in accordance with their natural and forced responses, and they may become unevenly distributed in a circumferential direction. This gives rise to an unbalance by shifting the center of gravity of the rotor away from its axis of rotation.
A blade that is offset from its equilibrium position oscillates about the equilibrium position at a frequency ωδ, which is the resonant frequency of the blade in its lead-lag motion, also referred to as its first lead-lag mode, or its resonant mode of lead-lag vibration. This frequency ωδ is relative to a rotary system of axes associated with the rotor, in application of definitions given below.
If Ω is the angular frequency of the rotation of the rotor, it is known that the fuselage of the helicopter is thus excited at the angular frequencies |Ω±ωδ|.
When standing on the ground via its landing gear, the fuselage of the helicopter constitutes a mass system that is suspended above the ground by a spring and a damper via each of its undercarriages. The fuselage standing on its landing gear thus presents resonant modes of vibration in roll and in pitching. There is a risk of instability on the ground when the excitation frequency of the rotor on the landing gear is close to the resonant frequency of oscillation |Ω±ωδ| or |Ω−ωδ| that corresponds to the phenomenon known as ground resonance.
In general, this phenomenon occurs in the presence of only the single resonant frequency of oscillation |Ω−ωδ| that is said to be “regressive”, since it relates to the so-called “regressive” resonant frequency of lead-lag vibration of the blades. This resonant frequency of oscillation is relative to a system of stationary axes associated with the rotorcraft, as explained below.
In order to avoid instability, it is known to begin by seeking to avoid these frequencies crossing each other, and/or, particularly if such a crossing is expected, it is known to provide sufficient damping of the fuselage on its landing gear and/or of the blades of the main rotor in their lead-lag motion. It is also possible to seek to have these frequencies cross at low speed.
Furthermore, other potential frequency crossings are observed in flight that lead to a resonance phenomenon at lower energy levels that is known as “air resonance”.
(2) Description of Related Art
Devices are already known for avoiding the above-mentioned instabilities in flight and for avoiding them in part on the ground with the rotor rotating, which devices are useful only during short stages of rotor operation.
For example, according to document FR 2 629 163, it is possible to arrange a resilient return strut on a rotor, which struts may be relatively complex.
According to document EP 0 340 095, a resilient return damper is arranged between two adjacent blades of a rotor.
Document EP 0 742 144 suggests using a lead-lag damper connected to a blade and to an adjacent blade, and also to the hub of the rotor via multiple levers.
Documents FR 2 890 937 and EP 1 767 452 seek to provide a rotorcraft rotor having a lead-lag damper system fastened to a lateral projection from each blade by means of a ball joint close to the pitch rod of the blade.
Document EP 2 233 397 describes an active device that acts on the lead-lag angle of a blade.
It should be observed that those devices making use of a plurality of dampers co-operating with the blades of a rotor may need to be matched. It will be understood that it is difficult or even impossible to fabricate dampers that are strictly identical. Under such circumstances, if the dampers of a device possess characteristics that are relatively far apart, those dampers may themselves give rise to unbalance being created.
The person skilled in the art therefore sometimes tends to match dampers in order to reduce that risk.
Furthermore, the dampers are stressed under dynamic conditions such that they are subjected to heating, thereby reducing their effectiveness and shortening their lifetime.
Thus, the movements of the blades of an articulated rotor of a rotorcraft tend to move the center of gravity of the rotor during each revolution. Such a movement is due to lead-lag movements of the blades under the effect of the natural response of the blades to an impact and the forced response of the blades to excitation taking place at a given frequency. In order to avoid the rare appearance of destructive phenomena of the ground resonance type or of the air resonance type, for example, the prior art includes passive devices that are relatively complicated and active devices that require energy to be supplied to them.
Document GB 947 683 discloses actuators that act on the movements of the blades.
Documents U.S. Pat. No. 5,372,478, U.S. Pat. No. 4,297,078, FR 2 373 445, and US 2009/180882 are also known.